U.S. patent application number 14/936094 was filed with the patent office on 2016-05-12 for battery system.
This patent application is currently assigned to TOYOTA JIDOSHA KABUSHIKI KAISHA. The applicant listed for this patent is TOYOTA JIDOSHA KABUSHIKI KAISHA. Invention is credited to Daiki TERASHIMA.
Application Number | 20160134139 14/936094 |
Document ID | / |
Family ID | 55803487 |
Filed Date | 2016-05-12 |
United States Patent
Application |
20160134139 |
Kind Code |
A1 |
TERASHIMA; Daiki |
May 12, 2016 |
BATTERY SYSTEM
Abstract
A battery system includes: an alkali secondary battery in which
a negative electrode open-circuit potential remains constant within
a predetermined SOC range; a temperature sensor configured to
detect a temperature of the battery; a current sensor; a voltage
sensor; and a controller configured to set an upper limit power
value that is discharged from the alkali secondary battery, the
controller being configured to: calculate a negative electrode
potential of the alkali secondary battery based on the negative
electrode open-circuit potential, a resistance value of a negative
electrode of the alkali secondary battery, the resistance value
being specified from the temperature, and the current value;
calculate a positive electrode potential of the alkali secondary
battery based on the negative electrode potential and the voltage
value; and reduce the upper limit power value below a reference
power value when the positive electrode potential is equal to or
lower than a threshold.
Inventors: |
TERASHIMA; Daiki;
(Toyota-shi, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
TOYOTA JIDOSHA KABUSHIKI KAISHA |
Toyota-shi |
|
JP |
|
|
Assignee: |
TOYOTA JIDOSHA KABUSHIKI
KAISHA
Toyota-shi
JP
|
Family ID: |
55803487 |
Appl. No.: |
14/936094 |
Filed: |
November 9, 2015 |
Current U.S.
Class: |
320/136 |
Current CPC
Class: |
H02J 7/0031 20130101;
Y02E 60/10 20130101 |
International
Class: |
H02J 7/00 20060101
H02J007/00 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 11, 2014 |
JP |
2014-229224 |
Claims
1. A battery system comprising: an alkali secondary battery in
which a negative electrode open-circuit potential remains constant
within a predetermined SOC range; a temperature sensor configured
to detect a temperature of the alkali secondary battery; a current
sensor configured to detect a current value of the alkali secondary
battery; a voltage sensor configured to detect a voltage value of
the alkali secondary battery; and a controller configured to set an
upper limit power value that is discharged from the alkali
secondary battery, the controller being configured to calculate a
negative electrode potential of the alkali secondary battery based
on the negative electrode open-circuit potential, a resistance
value of a negative electrode of the alkali secondary battery, the
resistance value being specified from the temperature, and the
current value, the controller being configured to calculate a
positive electrode potential of the alkali secondary battery based
on the negative electrode potential and the voltage value, and the
controller being configured to reduce the upper limit power value
below a reference power value when the positive electrode potential
is equal to or lower than a threshold.
2. The battery system according to claim 1, wherein the controller
is configured to calculate a damage amount defining deterioration
of a positive electrode of the alkali secondary battery based on
the positive electrode potential and the current value, and
calculate an integrated damage amount by integrating the damage
amount, the controller is configured to modify the threshold in
accordance with the integrated damage amount such that the
threshold decreases steadily as the integrated damage amount
increases, and the controller is configured to reduce the upper
limit power value below the reference power value when the positive
electrode potential is equal to or lower than the threshold
corresponding to the integrated damage amount.
3. The battery system according to claim 2, wherein the threshold
includes a first threshold set in advance and a second threshold
corresponding to the integrated damage amount, the controller is
configured to set the upper limit power value at the reference
power value until the positive electrode potential falls to or
below the second threshold between connecting the alkali secondary
battery to a load and disconnecting the alkali secondary battery
from the load, the controller is configured to reduce the upper
limit power value below the reference power value when the positive
electrode potential falls to or below the second threshold before
the alkali secondary battery is disconnected from the load, and the
controller is configured to reduce the upper limit power value
below the reference power value when the positive electrode
potential falls to or below the first threshold after the upper
limit power value is reduced below the reference power value.
Description
INCORPORATION BY REFERENCE
[0001] The disclosure of Japanese Patent Application No.
2014-229224, filed on Nov. 11, 2014 including the specification,
drawings and abstract is incorporated herein by reference in its
entirety.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The invention relates to a battery system in which discharge
of an alkali secondary battery is controlled on the basis of a
positive electrode potential of the alkali secondary battery.
[0004] 2. Description of Related Art
[0005] In International Patent Application No. 2013/105140 (W/O
2013/105140), the occurrence of a secondary reaction in a positive
electrode (variation in a structure or a crystallinity of the
positive electrode) is suppressed by measuring a positive electrode
potential using a reference electrode, and reducing an upper limit
power that can be output (discharged) from a single cell when the
positive electrode potential is lower than a threshold potential.
W/O 2013/105140 also describes use of a battery model method to
estimate the positive electrode potential.
[0006] When a reference electrode is used, as in W/O 2013/105140,
the positive electrode potential can be measured, but when the
reference electrode is omitted, the positive electrode potential
cannot be measured.
[0007] However, when a battery model method is used, as described
in W/O 2013/105140, the positive electrode potential can be
estimated without the use of a reference electrode. In an alkali
secondary battery, a memory effect occurs, and the positive
electrode potential of the alkali secondary battery is dependent on
a state of charge (SOC) and the memory effect of the secondary
battery. In other words, a voltage value and the positive electrode
potential of the alkali secondary battery vary due to the memory
effect even when the SOC of the alkali secondary battery remains
identical.
[0008] In the battery model method described in W/O 2013/105140,
the memory effect of the alkali secondary battery is not taken into
account, and therefore room for improvement remains with respect to
estimation of the positive electrode potential of the alkali
secondary battery.
SUMMARY OF THE INVENTION
[0009] A battery system according to an aspect of the invention
includes: an alkali secondary battery in which a negative electrode
open-circuit potential remains constant within a predetermined SOC
range; a temperature sensor configured to detect a temperature of
the alkali secondary battery; a current sensor configured to detect
a current value of the alkali secondary battery; a voltage sensor
configured to detect a voltage value of the alkali secondary
battery; and a controller configured to set an upper limit power
value that can be discharged from the alkali secondary battery. The
controller is configured to calculate a negative electrode
potential of the alkali secondary battery on the basis of the
negative electrode open-circuit potential, a resistance value of a
negative electrode of the alkali secondary battery, the resistance
value being specified from the temperature, and the current value,
and configured to calculate a positive electrode potential of the
alkali secondary battery on the basis of the negative electrode
potential and the voltage value. Further, the controller is
configured to reduce the upper limit power value below a reference
power value when the positive electrode potential is equal to or
lower than a threshold.
[0010] According to this aspect of the invention, the positive
electrode potential is calculated on the basis of the negative
electrode potential and the voltage value after calculating the
negative electrode potential. The voltage value and the negative
electrode potential are affected by a memory effect, and therefore,
by calculating the positive electrode potential from the voltage
value and the negative electrode potential, the positive electrode
potential as affected by the memory effect can be learned.
[0011] Here, when polarization is unlikely to occur in the alkali
secondary battery, variation in the negative electrode potential
due to polarization is also unlikely to occur, and therefore the
negative electrode potential can be calculated on the basis of the
negative electrode open-circuit potential, the current value, and
the resistance value of the negative electrode. The negative
electrode open-circuit potential remains constant regardless of the
SOC of the alkali secondary battery, and therefore the negative
electrode potential can be calculated without taking the SOC of the
alkali secondary battery into consideration.
[0012] By reducing the upper limit power value below the reference
power value when the positive electrode potential is equal to or
lower than the threshold, discharge of the alkali secondary battery
can be restricted more easily, and as a result, deterioration of a
positive electrode of the alkali secondary battery can be
suppressed. Elution of a conductive material contained in the
positive electrode may be cited as an example of deterioration of
the positive electrode, and the threshold is set in consideration
of deterioration of the positive electrode.
[0013] The controller may calculate a damage amount defining
deterioration of the positive electrode of the alkali secondary
battery on the basis of the positive electrode potential and the
current value, and calculate an integrated damage amount by
integrating the damage amount. Further, the controller may modify
the threshold in accordance with the integrated damage amount so
that the threshold decreases steadily as the integrated damage
amount increases. Then, when the positive electrode potential is
equal to or lower than the threshold corresponding to the
integrated damage amount, the controller can reduce the upper limit
power value below the reference power value. Here, the upper limit
power value may be maintained at the reference power value until
the positive electrode potential falls to or below the threshold
corresponding to the integrated damage amount.
[0014] By reducing the upper limit power value below the reference
power value when the positive electrode potential falls to or below
the threshold corresponding to the integrated damage amount,
discharge of the alkali secondary battery can be restricted more
easily, and as a result, deterioration of the positive electrode of
the alkali secondary battery can be suppressed. Further, when the
integrated damage amount increases, deterioration of the positive
electrode advances, and as a result, an output of the alkali
secondary battery may become insufficient. Hence, by maintaining
the upper limit power value at the reference power value until the
positive electrode potential falls to or below the threshold
corresponding to the integrated damage amount, the output of the
alkali secondary battery can be secured more easily.
[0015] The threshold may include a first threshold set in advance
and a second threshold corresponding to the integrated damage
amount. Here, between connecting the alkali secondary battery to a
load and disconnecting the alkali secondary battery from the load,
the controller may set the upper limit power value at the reference
power value until the positive electrode potential falls to or
below the second threshold, and reduce the upper limit power value
below the reference power value when the positive electrode
potential falls to or below the second threshold. After the upper
limit power value is thus reduced below the reference power value,
the controller may reduce the upper limit power value below the
reference power value when the positive electrode potential falls
to or below the first threshold.
[0016] By setting the upper limit power value at the reference
power value until the positive electrode potential falls to or
below the second threshold, the output of the alkali secondary
battery can be secured more easily, as described above. Further, by
reducing the upper limit power value below the reference power
value when the positive electrode potential falls to or below the
second threshold, deterioration of the positive electrode of the
alkali secondary battery can be suppressed, as described above.
Furthermore, by reducing the upper limit power value below the
reference power value when the positive electrode potential falls
to or below the first threshold after reducing the upper limit
power value below the reference power value, suppressing
deterioration of the positive electrode of the alkali secondary
battery can be prioritized.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] Features, advantages, and technical and industrial
significance of exemplary embodiments of the invention will be
described below with reference to the accompanying drawings, in
which like numerals denote like elements, and wherein:
[0018] FIG. 1 is a view showing a configuration of a battery
system;
[0019] FIG. 2 is a flowchart showing processing for setting a
discharge power allowable value, according to a first
embodiment;
[0020] FIG. 3 is a view showing a relationship between an SOC and a
negative electrode open-circuit potential of a secondary
battery;
[0021] FIG. 4 is a view showing variation in a negative electrode
potential when the secondary battery is discharged;
[0022] FIG. 5 is a view showing variation in the negative electrode
potential when the secondary battery is discharged;
[0023] FIG. 6 is a view illustrating elution of a conductive
material in a positive electrode active material layer;
[0024] FIG. 7 is a view showing a correspondence relationship
between a reference power value and a battery temperature during
discharge;
[0025] FIG. 8 is a view showing a correspondence relationship
between the reference power value and the SOC during discharge;
[0026] FIG. 9 is a flowchart showing processing for setting the
discharge power allowable value, according to a second
embodiment;
[0027] FIG. 10 is a view showing a correspondence relationship
between a positive electrode potential and a damage amount; and
[0028] FIG. 11 is a view showing a correspondence relationship
between an integrated damage amount and a threshold.
DETAILED DESCRIPTION OF EMBODIMENTS
[0029] Embodiments of the invention will be described below.
First Embodiment
[0030] A battery system according to a first embodiment will be
described using FIG. 1. A secondary battery 10 is connected to a
load 20 via a positive electrode line PL and a negative electrode
line NL. An alkali secondary battery such as a nickel hydrogen
battery is used as the secondary battery 10. A memory effect occurs
in an alkali secondary battery.
[0031] The secondary battery 10 includes a positive electrode
plate, a negative electrode plate, and a separator disposed between
the positive electrode plate and the negative electrode plate. The
positive electrode plate includes a collector plate and a positive
electrode active material layer formed on a surface of the
collector plate, and the positive electrode active material layer
includes a positive electrode active material and a conductive
material. The negative electrode plate includes a collector plate
and a negative electrode active material layer formed on a surface
of the collector plate, and the negative electrode active material
layer includes a negative electrode active material and a
conductive material. The positive electrode active material layer,
the negative electrode active material layer, and the separator are
impregnated with an electrolyte.
[0032] A system main relay SMR-B is provided on the positive
electrode line PL, and a system main relay SMR-G is provided on the
negative electrode line NL. The system main relays SMR-B, SMR-G are
switched ON and OFF by control signals received from a controller
40.
[0033] The load 20 operates upon reception of power discharged from
the secondary battery 10, and supplies power (charging power) to
the secondary battery 10. When the battery system according to this
embodiment is installed in a vehicle, a motor/generator can be used
as the load 20. The motor/generator generates kinetic energy for
causing the vehicle to travel upon reception of the power
discharged from the secondary battery 10. Further, the
motor/generator is capable of converting kinetic energy generated
when the vehicle brakes into power, and outputting this power
(regenerated power) to the secondary battery 10.
[0034] Note that when the secondary battery 10 is installed in a
vehicle, a battery pack formed by connecting a plurality of
secondary batteries 10 in series may be installed in the vehicle.
Here, the battery pack may include a plurality of secondary
batteries 10 connected in parallel. The battery pack may be formed
from a plurality of battery blocks connected in series. Here, each
battery block is constituted by a plurality of secondary batteries
(single cells) 10 connected in series.
[0035] A voltage sensor 31 detects a voltage value Vb of the
secondary battery 10, and outputs a detection result to the
controller 40. When the battery pack described above is used, the
voltage sensor 31 may detect the voltage value of the
aforementioned battery block. In this case, the voltage value of
the secondary battery 10 can be calculated by dividing the voltage
value of the battery block by the number of secondary batteries 10
constituting the battery block.
[0036] A current sensor 32 detects a current value Ib of the
secondary battery 10, and outputs a detection result to the
controller 40. In this embodiment, the current value Ib during
discharge of the secondary battery 10 is set to take a positive
value, and the current value Ib during charging of the secondary
battery 10 is set to take a negative value. A temperature sensor 33
detects a temperature (to be referred to as a battery temperature)
Tb of the secondary battery 10, and outputs a detection result to
the controller 40. The controller 40 includes a memory 41 that
stores predetermined information.
[0037] Next, processing for setting a discharge power allowable
value Wout in the battery system according to this embodiment will
be described using a flowchart shown in FIG. 2. The processing
shown in FIG. 2 is executed at predetermined period intervals when
the system main relays SMR-B, SMR-G are ON.
[0038] The discharge power allowable value Wout is an upper limit
power value that can be discharged from the secondary battery 10.
When the secondary battery 10 is discharged, the controller 40
controls discharge of the secondary battery 10 so that a discharge
power value of the secondary battery 10 does not exceed the
discharge power allowable value Wout. As a result, the discharge
power value of the secondary battery 10 varies within a range no
higher than the discharge power allowable value Wout.
[0039] In step S101, the controller 40 detects the voltage value
Vb, the current value Ib, and the battery temperature Tb using the
voltage sensor 31, the current sensor 32, and the temperature
sensor 33. In step S102, the controller 40 calculates a negative
electrode potential En of the secondary battery 10. More
specifically, the controller 40 calculates the negative electrode
potential En on the basis of Equation (1), shown below.
En=OCV_n+Ib.times.R (1)
[0040] In Equation (1), OCV_n denotes an open-circuit potential of
the negative electrode, and R denotes a resistance value of the
negative electrode. The open-circuit potential OCV_n remains
constant within a predetermined SOC range of the secondary battery
10, or in other words remains constant regardless of the SOC of the
secondary battery 10. The predetermined SOC range is a possible
range of the SOC of the secondary battery 10 when
charging/discharging of the secondary battery 10 is controlled. The
open-circuit potential OCV_n may therefore be determined in
advance.
[0041] The resistance value R is dependent on the battery
temperature Tb, and therefore a correspondence relationship between
the resistance value R and the battery temperature Tb can be
determined in advance (in the form of a map or a calculation
formula). By detecting the battery temperature Tb using this
correspondence relationship, the resistance value R corresponding
to the detected battery temperature Tb can be specified.
[0042] Correspondence relationships of the open-circuit potential
OCV_n with the resistance value R and the battery temperature Tb
may be stored in the memory 41 in advance. By inserting the
open-circuit potential OCV_n determined in advance, the resistance
value R corresponding to the battery temperature Tb detected in the
processing of step S101, and the current value Ib detected in the
processing of step S101 into Equation (1), the negative electrode
potential En can be calculated.
[0043] As shown in FIG. 3, the open-circuit potential OCV_n of the
negative electrode remains fixed at a value lower than 0 [V]
regardless of the SOC of the secondary battery 10. In FIG. 3, the
abscissa shows the SOC of the secondary battery 10, and the
ordinate shows the negative electrode potential En.
[0044] As shown in FIG. 4, when the secondary battery 10 is
discharged, the negative electrode potential En becomes higher than
the open-circuit potential OCV_n. In FIG. 4, the abscissa shows
time and the ordinate shows the negative electrode potential En.
Here, a difference between the negative electrode potential En and
the open-circuit potential OCV_n during discharge corresponds to an
amount of variation in the negative electrode potential En while
the secondary battery 10 is energized, and takes a value obtained
by multiplying the current value Ib by the resistance value R.
Therefore, as shown in Equation (1), the negative electrode
potential En can be calculated on the basis of the open-circuit
potential OCV_n, the current value Ib, and the resistance value
R.
[0045] As the battery temperature Tb increases, the negative
electrode potential En becomes steadily more likely to exhibit the
behavior shown in FIG. 4. More specifically, when the battery
temperature Tb equals or exceeds a predetermined temperature Tb_th
during discharge of the secondary battery 10, polarization does not
occur, and therefore, as shown in FIG. 4, the negative electrode
potential En exhibits a constant potential regardless of the
discharge time.
[0046] When the battery temperature Tb is lower than the
predetermined temperature Tb_th, on the other hand, polarization is
more likely to occur, and therefore, as shown in FIG. 5, the
negative electrode potential En is more likely to vary in
accordance with the discharge time. In FIG. 5, the abscissa shows
time and the ordinate shows the negative electrode potential En. As
shown in FIG. 5, when the negative electrode potential En varies,
it becomes more difficult to specify the negative electrode
potential En.
[0047] When the secondary battery 10 is connected to the load 20
and subjected to charging/discharging, the battery temperature Tb
typically reaches or exceeds the predetermined temperature Tb_th
easily due to heat generated in the secondary battery 10 while the
secondary battery 10 is energized. Polarization is therefore
unlikely to occur while the secondary battery 10 is charged and
discharged, and as a result, the negative electrode potential En is
more likely to exhibit a constant potential regardless of the
discharge time. Hence, the negative electrode potential En can be
calculated on the basis of Equation (1).
[0048] In step S103, the controller 40 calculates a positive
electrode potential Ep of the secondary battery 10. More
specifically, the controller 40 calculates the positive electrode
potential Ep on the basis of Equation (2), shown below. The voltage
value Vb corresponds to a difference between the positive electrode
potential Ep and the negative electrode potential En, from which
Equation (2) can be derived.
Ep=Vb+En (2)
[0049] In Equation (2), the voltage value Vb detected in the
processing of step S101 is used as the voltage value Vb, and the
negative electrode potential En calculated in the processing of
step S102 is used as the negative electrode potential En.
[0050] In step S104, the controller 40 determines whether or not
the positive electrode potential Ep calculated in the processing of
step S103 is equal to or lower than a threshold (a positive
electrode potential) Ep_th. The threshold Ep_th is a fixed value
set in advance in consideration of a potential at which the
positive electrode plate of the secondary battery 10 deteriorates,
and is set at a larger potential than 0 [V]. Elution of the
conductive material in the positive electrode active material layer
into the electrolyte may be cited as an example of deterioration of
the positive electrode plate.
[0051] As shown in FIG. 6, a positive electrode active material
layer 112 constituted by a positive electrode active material 112a
and a conductive material 112b is formed on a surface of a
collector plate 111 of a positive electrode plate 11, and a surface
of the positive electrode active material 112a is covered by the
conductive material 112b. When the positive electrode potential Ep
falls to or below the threshold Ep_th, the conductive material 112b
is eluted into the electrolyte, thereby forming eluate 112c. Once
the eluate 112c is formed, the eluate 112c cannot return to the
conductive material 112b. As the amount of eluate 112c increases,
therefore, the amount of conductive material 112b decreases,
leading to a reduction in conductivity in the positive electrode
plate 11. When the conductivity of the positive electrode plate 11
decreases, an output performance of the secondary battery 10
deteriorates.
[0052] When the positive electrode potential Ep is equal to or
lower than the threshold Ep_th in the processing of step S104 shown
in FIG. 2, the controller 40 sets the discharge power allowable
value Wout at a smaller value than a reference power value Wout_ref
in step S105. The reference power value Wout_ref will be described
below. Here, a difference between the set discharge power allowable
value Wout and the reference power value Wout_ref is expressed as
.DELTA.Wout.
[0053] The difference .DELTA.Wout may be a fixed value determined
in advance, or may be modified in accordance with the difference
between the positive electrode potential Ep and the threshold
Ep_th. When the difference .DELTA.Wout is modified in accordance
with the difference between the positive electrode potential Ep and
the threshold Ep_th, the difference .DELTA.Wout can be increased as
the difference between the positive electrode potential Ep and the
threshold Ep_th increases. In other words, the difference
.DELTA.Wout can be reduced as the difference between the positive
electrode potential Ep and the threshold Ep_th decreases.
[0054] Here, the difference .DELTA.Wout may be modified either
continuously or in steps in accordance with the difference between
the positive electrode potential Ep and the threshold Ep_th.
Modifying the difference .DELTA.Wout in steps includes not
modifying the difference .DELTA.Wout even when the difference
between the positive electrode potential Ep and the threshold Ep_th
varies. When the difference .DELTA.Wout is modified continuously,
the difference .DELTA.Wout is always modified in response to
variation in the difference between the positive electrode
potential Ep and the threshold Ep_th.
[0055] When the positive electrode potential Ep is higher than the
threshold Ep_th in the processing of step S104 shown in FIG. 2, the
controller 40 sets the discharge power allowable value Wout at the
reference power value Wout_ref in step S106.
[0056] The reference power value Wout_ref is set on the basis of at
least one of the battery temperature Tb and the SOC of the
secondary battery 10. This will now be described more
specifically.
[0057] FIG. 7 shows a correspondence relationship between the
reference power value Wout_ref and the battery temperature Tb. As
shown in FIG. 7, by determining the correspondence relationship
between the reference power value Wout_ref and the battery
temperature Tb in advance (in the form of a map or a calculation
formula) and then detecting the battery temperature Tb, the
reference power value Wout_ref corresponding to the detected
battery temperature Tb can be set. The correspondence relationship
between the reference power value Wout_ref and the battery
temperature Tb may be stored in the memory 41 in advance.
[0058] In FIG. 7, when the battery temperature Tb is equal to or
higher than a first predetermined temperature Tb_th1 and equal to
or lower than a second predetermined temperature Tb_th2, the
reference power value Wout_ref takes a predetermined value (a fixed
value). In other words, when the battery temperature Tb is equal to
or higher than the first predetermined temperature Tb_th1 and equal
to or lower than the second predetermined temperature Tb_th2, the
reference power value Wout_ref takes the predetermined value
regardless of the battery temperature Tb. Note that the second
predetermined temperature Tb_th2 is higher than the first
predetermined temperature Tb_th1.
[0059] When the battery temperature Tb is lower than the first
predetermined temperature Tb_th1, on the other hand, the reference
power value Wout_ref decreases as the battery temperature Tb
decreases. Further, when the battery temperature Tb is higher than
the second predetermined temperature Tb_th2, the reference power
value Wout_ref decreases as the battery temperature Tb increases.
Depending on the battery temperature Tb, the reference power value
Wout_ref may be set at 0 [kW], and at this time the secondary
battery 10 is not discharged.
[0060] FIG. 8 shows a correspondence relationship between the
reference power value Wout_ref and the SOC of the secondary battery
10. As shown in FIG. 8, by determining the correspondence
relationship between the reference power value Wout_ref and the SOC
of the secondary battery 10 in advance (in the form of a map or a
calculation formula) and then calculating the SOC of the secondary
battery 10, the reference power value Wout_ref corresponding to the
calculated SOC can be set. The correspondence relationship between
the reference power value Wout_ref and the SOC of the secondary
battery 10 may be stored in the memory 41 in advance. The SOC of
the secondary battery 10 can be calculated on the basis of the
current value Ib and the voltage value Vb as in the conventional
method.
[0061] In FIG. 8, when the SOC of the secondary battery 10 is equal
to or higher than a threshold SOC_th, the reference power value
Wout_ref takes a predetermined value regardless of the SOC. When
the SOC of the secondary battery 10 is lower than the threshold
SOC_th, on the other hand, the reference power value Wout_ref
decreases as the SOC decreases. Here, depending on the SOC of the
secondary battery 10, the reference power value Wout_ref may be set
at 0 [kW], and at this time the secondary battery 10 is not
discharged.
[0062] To set the reference power value Wout_ref on the basis of
the battery temperature Tb and the SOC of the secondary battery 10,
the correspondence relationships of the reference power value
Wout_ref with the battery temperature Tb and the SOC are preferably
determined in advance (in the form of maps or calculation
formulae). Thus, by detecting the battery temperature Tb and
calculating the SOC of the secondary battery 10, the reference
power value Wout_ref corresponding to the detected battery
temperature Tb and the calculated SOC can be set.
[0063] Actions and effects of this embodiment will now be
described.
[0064] According to this embodiment, by reducing the discharge
power allowable value Wout below the reference power value Wout_ref
when the positive electrode potential Ep is equal to or lower than
the threshold Ep_th, discharge of the secondary battery 10 can be
restricted more easily, and as a result, charging of the secondary
battery 10 is prioritized. By charging the secondary battery 10,
the positive electrode potential Ep can be increased, and as a
result, the positive electrode potential Ep can be varied to a
higher potential than the threshold Ep_th.
[0065] As described using FIG. 6, when the positive electrode
potential Ep remains at or below the threshold Ep_th, the
conductive material 112b of the positive electrode active material
layer 112 is continuously eluted into the electrolyte, but by
increasing the positive electrode potential Ep to a higher
potential than the threshold Ep_th, elution of the conductive
material 112b can be suppressed. As a result, a reduction in the
output performance of the secondary battery 10 due to elution of
the conductive material 112b can be suppressed.
[0066] When the discharge power allowable value Wout is set on the
basis of the positive electrode potential Ep, as in this
embodiment, the positive electrode potential Ep may be calculated
directly. However, the positive electrode potential Ep varies in
accordance with the SOC and the memory effect of the secondary
battery 10. Moreover, even when the SOC of the secondary battery 10
remains constant, the positive electrode potential Ep varies in
accordance with a generation condition of the memory effect. Since
it is difficult to grasp the generation condition of the memory
effect, it is also difficult to calculate the positive electrode
potential Ep directly.
[0067] In this embodiment, the negative electrode potential En is
calculated before calculating the positive electrode potential Ep.
This embodiment focuses on the fact that the open-circuit potential
OCV_n of the negative electrode remains constant regardless of the
SOC of the secondary battery 10. This embodiment also focuses on
the fact that during discharge of the secondary battery 10, the
negative electrode potential En does not vary regardless of the
discharge time, and therefore the negative electrode potential En
is not affected by polarization. In consideration of these facts,
the negative electrode potential En can be calculated on the basis
of Equation (1).
[0068] After calculating the negative electrode potential En, the
positive electrode potential Ep can be calculated on the basis of
the voltage value Vb and the negative electrode potential En, as
shown in Equation (2). Here, the voltage value Vb and the positive
electrode potential Ep are affected by the memory effect, and
therefore, by calculating the positive electrode potential Ep from
the voltage value Vb and the negative electrode potential En, the
positive electrode potential Ep as affected by the memory effect
can be learned.
[0069] A battery system according to a second embodiment of the
invention will now be described. In this embodiment, identical
constituent elements to constituent elements described in the first
embodiment have been allocated identical reference symbols, and
detailed description thereof has been omitted.
[0070] In the first embodiment, the discharge power allowable value
Wout is reduced below the reference power value Wout_ref when the
positive electrode potential Ep is equal to or lower than the
(fixed value) threshold Ep_th. In this embodiment, the threshold
Ep_th (to be referred to as a threshold Ep_th_ini) used when the
processing for reducing the discharge power allowable value Wout
below the reference power value Wout_ref is performed for the first
time between connecting the secondary battery 10 to the load 20 and
disconnecting the secondary battery 10 from the load 20 is
modified. Here, the threshold Ep_th corresponds to a first
threshold according to the invention, while the threshold Ep_th_ini
corresponds to a second threshold according to the invention.
[0071] In this embodiment, a deterioration condition of the
positive electrode plate 11 (for example, an elution condition of
the conductive material 112b) is defined as a damage amount D, and
the threshold Ep_th_ini is modified on the basis of an integrated
value of the damage amount D (to be referred to as an integrated
damage amount .SIGMA.D). The integrated damage amount .SIGMA.D is
an integrated value of the damage amount D from the first time the
secondary battery 10 is subjected to charging/discharging to the
present.
[0072] When the integrated damage amount .SIGMA.D is zero, the
threshold Ep_th_ini is equal to the (fixed value) threshold Ep_th.
When the integrated damage amount .SIGMA.D is larger than zero, the
threshold Ep_th_ini is lower than the (fixed value) threshold
Ep_th. Here, a difference between the threshold Ep_th_ini and the
(fixed value) threshold Ep_th widens as the integrated damage
amount .SIGMA.D increases.
[0073] Processing for setting the discharge power allowable value
Wout according to this embodiment will be described using a
flowchart shown in FIG. 9. The processing shown in FIG. 9 is
executed at predetermined period intervals when the system main
relays SMR-B, SMR-G are ON. In FIG. 9, identical processes to the
processes described using FIG. 2 have been allocated identical step
numbers, and detailed description thereof has been omitted.
[0074] After calculating the positive electrode potential Ep in the
processing of step S103, the controller 40 determines whether or
not a discharge restriction flag is set in step S107. The setting
condition of the discharge restriction flag is stored in the memory
41.
[0075] Here, the controller 40 cancels setting of the discharge
restriction flag every time the secondary battery 10 and the load
20 are disconnected, or in other words every time the system main
relays SMR-B, SMR-G are switched OFF. The discharge restriction
flag is therefore not set when the secondary battery 10 is
connected to the load 20, or in other words when the system main
relays SMR-B, SMR-G are switched ON. Furthermore, as will be
described below, the not set until the secondary battery 10 is
connected to the load 20 and the discharge power allowable value
Wout is reduced below the reference power value Wout_ref.
[0076] When the discharge restriction flag is not set in the
processing of step S107, the controller 40 calculates the damage
amount D in step S108 on the basis of the positive electrode
potential Ep calculated in the processing of step S103 and the
current value Ib detected in the processing of step S101. More
specifically, the damage amount D is calculated as shown in FIG.
10. In FIG. 10, the abscissa shows the positive electrode potential
Ep and the ordinate shows the damage amount D.
[0077] When the positive electrode potential Ep is higher than the
(fixed value) threshold Ep_th, the damage amount D is zero. The
damage amount D is also zero when charging is underway in the
secondary battery 10 and when charging/discharging is not underway
in the secondary battery 10.
[0078] When the positive electrode potential Ep is equal to or
lower than the (fixed value) threshold Ep_th while discharging is
underway in the secondary battery 10, on the other hand, the damage
amount D increases from zero. Here, the damage amount D increases
as the positive electrode potential Ep decreases below the (fixed
value) threshold Ep_th, and decreases as the positive electrode
potential Ep approaches the (fixed value) threshold Ep_th. Further,
the damage amount D increases as the current value Ib during
discharge of the secondary battery 10 increases, and decreases as
the current value Ib during discharge of the secondary battery 10
decreases.
[0079] In step S109, the controller 40 calculates the current
integrated damage amount .SIGMA.D on the basis of the damage amount
D calculated in the processing of step S108. More specifically, the
controller 40 calculates the current integrated damage amount
.SIGMA.D by adding the damage amount D calculated most recently in
the processing of step S108 to the previously calculated integrated
damage amount .SIGMA.D. Further, the controller 40 stores the
current integrated damage amount .SIGMA.D in the memory 41. The
integrated damage amount .SIGMA.D stored in the memory 41 is used
to calculate the next integrated damage amount .SIGMA.D.
[0080] In step S110, the controller 40 sets the threshold Ep_th_ini
corresponding to the integrated damage amount .SIGMA.D calculated
in the processing of step S109. More specifically, as shown in FIG.
11, the threshold Ep_th_ini corresponding to the integrated damage
amount .SIGMA.D can be set by determining a correspondence
relationship between the integrated damage amount .SIGMA.D and the
threshold Ep_th_ini in advance (in the form of a map or a
calculation formula). As shown in FIG. 11, the threshold Ep_th_ini
is set to be steadily lower than the (fixed value) threshold Ep_th
as the integrated damage amount .SIGMA.D increases. In other words,
the threshold Ep_th_ini steadily approaches the (fixed value)
threshold Ep_th as the integrated damage amount .SIGMA.D approaches
zero. When the integrated damage amount .SIGMA.D is zero, the
threshold Ep_th_ini is equal to the (fixed value) threshold
Ep_th.
[0081] In step S111, the controller 40 determines whether or not
the positive electrode potential Ep calculated in the processing of
step S103 is equal to or lower than the threshold Ep_th_ini set in
the processing of step S110. When the positive electrode potential
Ep is higher than the threshold Ep_th_ini, the controller 40
performs the processing of step S106.
[0082] When the positive electrode potential Ep is equal to or
lower than the threshold Ep_th_ini in the processing of step S111,
on the other hand, the controller 40 reduces the discharge power
allowable value Wout below the reference power value Wout_ref in
step S112. The processing of step S112 is identical to the
processing of step S105, described using FIG. 2.
[0083] In the processing of step S112, the difference .DELTA.Wout
may be a fixed value determined in advance, or may be modified in
accordance with the difference between the positive electrode
potential Ep and the threshold Ep_th_ini. When the difference
.DELTA.Wout is modified in accordance with the difference between
the positive electrode potential Ep and the threshold Ep_th_ini,
the difference .DELTA.Wout can be increased as the difference
between the positive electrode potential Ep and the threshold
Ep_th_ini increases. In other words, the difference .DELTA.Wout can
be reduced as the difference between the positive electrode
potential Ep and the threshold Ep_th_ini decreases.
[0084] Following the processing of step S112, the controller 40
sets the discharge restriction flag in step S113. The setting of
the discharge restriction flag is stored in the memory 41.
[0085] When the discharge restriction flag is set in the processing
of step S107, the controller 40 performs the processing of step
S104. The processing of steps S104, S105, and S106 is as described
in the first embodiment.
[0086] According to this embodiment, before the discharge
restriction flag is set, the threshold Ep_th_ini is set in
accordance with the integrated damage amount .SIGMA.D, and when the
positive electrode potential Ep is equal to or lower than the
threshold Ep_th_ini, the discharge power allowable value Wout is
reduced below the reference power value Wout_ref. Once the
discharge power allowable value Wout has been reduced below the
reference power value Wout_ref, the discharge restriction flag is
set.
[0087] After the discharge restriction flag is set, a determination
is made as to whether or not the positive electrode potential Ep is
equal to or lower than the (fixed value) threshold Ep_th. When the
positive electrode potential Ep is equal to or lower than the
(fixed value) threshold Ep_th, the discharge power allowable value
Wout is reduced below the reference power value Wout_ref.
[0088] As the integrated damage amount .SIGMA.D increases,
deterioration of the positive electrode plate 11 (elution of the
conductive material 112b, for example) advances, leading to a
reduction in the output performance of the secondary battery 10, as
described in the first embodiment. Here, by reducing the discharge
power allowable value Wout below the reference power value Wout_ref
when the positive electrode potential Ep is equal to or lower than
the (fixed value) threshold Ep_th, the output of the secondary
battery 10 may become insufficient. In particular, when the battery
system according to this embodiment is installed in a vehicle such
that the vehicle uses the output of the secondary battery 10, a
travel performance of the vehicle may deteriorate.
[0089] Hence, in this embodiment, the discharge power allowable
value Wout is held at the reference power value Wout_ref until the
positive electrode potential Ep falls to or below the threshold
Ep_th_ini in order to secure the output of the secondary battery
10.
[0090] By reducing the discharge power allowable value Wout below
the reference power value Wout_ref when the positive electrode
potential Ep is equal to or lower than the threshold Ep_th_ini, on
the other hand, charging of the secondary battery 10 can be
prioritized so that the positive electrode potential Ep can be
increased, and as a result, deterioration of the positive electrode
plate 11 (elution of the conductive material 112b, for example) can
be suppressed. Furthermore, after the discharge restriction flag is
set, the positive electrode potential Ep can be increased by
reducing the discharge power allowable value Wout below the
reference power value Wout_ref when the positive electrode
potential Ep falls to or below the (fixed value) threshold Ep_th,
and as a result, deterioration of the positive electrode plate 11
can be suppressed. In other words, after the discharge restriction
flag is set, suppressing deterioration of the positive electrode
plate 11 is prioritized.
* * * * *